A polycrystalline body comprising a ceramic (a polycrystalline ceramic body) is used, for example, in sensors for temperature, heat, gas, ions and the like, electron circuit parts such as capacitors, resistors and integrated circuit boards, and optical or magnetic recording elements. In particular, a polycrystalline ceramic body having a piezoelectric effect (hereinafter referred to as a “piezoelectric ceramic”) is being widely used in the field of electronics or mechatronics because of its high performance, high degree of freedom in shape and relatively easy material design.
The piezoelectric ceramic is obtained by subjecting a ferroelectric ceramic to a so-called polarizing process of applying an electric field to align the direction of polarization of the ferroelectric material to a fixed direction. In order to align spontaneous polarization to a fixed direction, by a polarizing process, in the piezoelectric ceramic, an isotropic perovskite-type crystal structure capable of spontaneously polarizing in three-dimensional direction is advantageous. Therefore, most piezoelectric ceramics in practical use are isotropic perovskite-type ferroelectric ceramics.
Known examples of isotropic perovskite-type ferroelectric ceramics include Pb(Zr,Ti)O3 (hereinafter referred to as “PZT”), a PZT ternary system obtained by adding a lead-based composite perovskite to PZT, BaTiO3 and Bi0.5Na0.5TiO3 (hereinafter referred to as “BNT”).
Among these, the lead-based piezoelectric ceramic as represented by PZT has good piezoelectric properties as compared with other piezoelectric ceramics and predominates in the piezoelectric ceramics practically used at present. However, this piezoelectric ceramic contains lead oxide (PbO) having a high vapor pressure and disadvantageously imposes a large load on the environment. Therefore, a low-lead or lead-free piezoelectric ceramic having piezoelectric properties equivalent to those of PZT is in demand.
On the other hand, a BaTiO3 ceramic has relatively high piezoelectric properties among lead-free piezoelectric materials and is being utilized in sonar systems and the like. Also, some solid solutions of BaTiO3 and another non-lead type perovskite compound (for example, BNT) show relatively good piezoelectric properties. However, these lead-free piezoelectric ceramics have a problem that the piezoelectric properties are inferior to those of PZT.
In order to solve such problems, various piezoelectric ceramics have been heretofore proposed.
For example, a piezoelectric ceramic comprising an isotropic perovskite-type potassium sodium niobate exhibiting relatively good piezoelectric properties, of non-lead materials, or a solid solution thereof, is known (see, Japanese Unexamined Patent Publication (Kokai) Nos. 2000-313664, 2003-300776, 2003-306479, 2003-327472, 2003-342069 and 2003-342071).
However, these lead-free piezoelectric ceramics have a problem that the piezoelectric properties are not yet satisfactory as compared with the PZT-type piezoelectric ceramic.
Under these circumstances, a piezoelectric element comprising a piezoelectric ceramic containing ceramic-crystal grains having shape anisotropy and a spontaneous polarization preferentially oriented in one plane is disclosed (see, Japanese Unexamined Patent Publication (Kokai) No. 2004-7406).
It is generally known that the piezoelectric properties, and the like, of the isotropic perovskite-type compound vary depending on the direction of crystal axis. Therefore, if the crystal axis giving good piezoelectric properties or the like can be oriented to a fixed direction, the anisotropy of piezoelectric properties can be maximally utilized and an enhancement in the performance of the piezoelectric ceramic can be expected. As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2004-7406 supra, according to a method of using, as a reactive template, a plate-like powder having a predetermined composition, and sintering the plate-like powder and a raw material powder, thereby orienting a specific crystal plane, a high-performance crystal-oriented ceramic in which a specific crystal plane has a high degree of orientation can be produced.
However, in the case of producing a polycrystalline ceramic body (crystal-oriented ceramic) by sintering a plate-like powder and a raw material powder as described above, a dense polycrystalline ceramic body is disadvantageously not obtained, because the plate-like powder and the raw material powder, which are different in the particle size, are sintered. Particularly, when a non-lead type material is used, there is a problem that voids are readily generated and densification scarcely occurs.
The polycrystalline ceramic body in which voids are generated is disadvantageous in that the piezoelectric properties such as the piezoelectric d31 constant, and the dielectric properties such as the dielectric loss, are deteriorated. Furthermore, there is a problem that the strength decreases and the polycrystalline ceramic body is readily fractured due to fatigue or the like in the course of use.
Generally, in the sintering of a ceramic, as the particle diameter of the powder particle used for the raw material becomes smaller, the specific surface becomes larger and the activity becomes higher, so that the sintering can be performed at a lower temperature as compared with the particle having a large particle diameter. Also, as the uniformity of the particle diameter of the raw material powder becomes higher, the sinterability becomes better and densification tends to be more easily attained.
As in the above-described technique for enhancing the crystal orientation of a ceramic, when a raw material powder prepared by blending a coarse template particle for accelerating the crystal orientation and a fine particle comprising a complementary substance for obtaining the objective ceramic is sintered, the sintering rate becomes non-uniform in the firing process due to a difference, in sinterability, between the fine particle and the coarse template particle.
For example, when the raw material powder is fired in the temperature region where sintering of the fine particle is accelerated, sintering of the coarse template particle with the fine particle can hardly proceed even though sintering of the fine particle is accelerated. Therefore, voids are formed in the periphery of the template particle and a dense sintered body may not be obtained. More specifically, in this case, a sintered body having a low open porosity may be obtained due to acceleration of sintering of the fine particle, but sintering of the template particle with the fine particle present in the periphery of the template particle is retarded and closed pores (voids) may remain in the sintered body. As a result, the sintered body obtained is decreased in the bulk density.
On the other hand, when firing is performed at a temperature higher than the temperature suitable for sintering, particle growth is accelerated rather than the sintering and this allows for occurrence of particle growth among fine particles and occurrence of sintering between the fine particle and the template particle and particle growth. Under this temperature condition, firm bonding is formed among the raw material powder particles such as fine powder and template powder, but the progress of sintering is retarded. As a result, the sintered body obtained may have increased open porosity.
Such a problem arises not only in the case of producing a polycrystalline ceramic body comprising a crystal-oriented ceramic but also in many other cases of producing a polycrystalline ceramic body.
That is, in the case of producing a polycrystalline ceramic body by mixing and sintering raw material powders differing in the particle diameter, there is a problem that densification can hardly occur.
The present invention has been made by taking account of these conventional problems and an object of the present invention is to provide a production method of a polycrystalline ceramic body with excellent density.